1. Field of the Invention
The present invention relates generally to FM demodulating apparatus and, more particularly, is directed to an FM demodulating apparatus suitable for use in a video tape recorder (VTR) to demodulate a video signal which is FM-modulated and then recorded.
2. Description of the Prior Art
A prior-art video tape recorder (VTR) utilizes an FM demodulating apparatus to FM-demodulate a reproduced signal of an FM luminance signal recorded on a magnetic tape. FIG. 1 shows an example of a previously-proposed FM demodulating apparatus.
Referring to FIG. 1, an input terminal 1 is supplied with a video signal which is FM-modulated by a carrier of a relatively low frequency and reproduced by a video tape recorder and the like. The FM-modulated video signal applied to the input terminal 1 is supplied to an FM demodulator 2 of a pulse count type and to a 90-degrees phase shifting circuit 3. The 90-degrees phase shifting circuit 3 shifts, i.e., delays the phase of the input video signal by 90 degrees. A video signal from the 90-degrees phase shifting circuit 3 is supplied to an FM demodulator 4 of a pulse count type. Demodulated signals from the two FM demodulators 2 and 4 are supplied to an adder 5, and an added signal from the adder 5 is supplied to a low-pass filter 6. Thus, the high frequency component of the added signal is filtered-out by the low-pass filter 6 and is then fed to an output terminal 7. Therefore, the FM-demodulated video signal is developed at the output terminal 7. In the FM-demodulated video signal at the output terminal 7, the occurrence of so-called moire, i.e., an undesirable high frequency produced around a frequency twice as high the carrier due to the side band wave, can be alleviated.
Recently, a video signal transmitting system is developed, in which a picture of high resolution, i.e., a high-definition television picture, is displayed. This high-definition television system processes a signal involving a high band component having a band width wider than that of the video signal according to the standard broadcasting system such as an NTSC system or the like. For this reason, when a high-definition television video signal is demodulated by the above-described FM demodulating apparatus, the undesirable high frequency component produced about the frequency twice the carrier frequency due to the side band wave cannot be suppressed satisfactorily.
As described above, this undesirable high frequency component produced about the frequency which is twice as high as the carrier frequency due to the side band wave is what might be called a moire. The moire occurs because the frequency characteristics of the input signal to the FM demodulator 2 and the input signal to the FM demodulator 4 are changed by the phase-shifting operation in the 90-degrees phase shifting circuit 3.